1. Field of Invention
The present disclosure relates to the field of new energy technology and the field of power electronic technology. More particularly, the present disclosure relates to a converter system applied in the area of new energy technology.
2. Description of Related Art
With the ever-increasing seriousness of energy-related issues in recent times, the development of new energy technology is gradually gaining the attention of those involved in research and development in various fields. While the capacity of power generation systems is becoming larger and larger, faced with capacity bottlenecks of modern electronic components, it is not easy to transmit a large amount of power energy to an electric grid through only a single converter.
In order to solve the problems mentioned above, the conventional parallel inverter technology, which can significantly increase the total current without increasing the current stress of a single power switch, is gradually becoming an active area of research and development. This has eventually led to research and development with respect to a converter product having a high power grade. In general, under a given input power, by using parallel technology, a power switch having a low power grade is used so as to reduce production costs. However, it should be pointed out that in a single converter system since there is no zero-sequence circulation channel, no problem of circular current is caused. Nevertheless, in multiple converter systems which are connected in parallel, once there is a circulation channel, a serious problem of circular current is caused. The circular current only flowing among parallel connected converters not only increases system loss, but also reduces system efficiency, so that a large amount of heat is generated in the switch and even more seriously the switch is burned. Furthermore, the circular current also causes unbalancing current, so that the current stress applied on the power switch is not balanced, which reduces the usage life of the power switch and limits the increased capacity of the total system. Moreover, the circular current also results in distortion of the three-phase current and increases the total harmonic distortion (THD), so that the grid connection of the system is not easily realized.
To eliminate the circular current of a converter system, a traditional solution involves eliminating the circulation channel using a hardware solution. For example, an isolation transformer may be arranged between a generator-side converter and a motor so as to eliminate the circulation channel. Also for example, when the DC buses of the two converters are connected in parallel, an isolation transformer may be arranged between the generator-side converter and the motor, and also between the grid-side converter and the grid. Another solution involves selecting a motor having a certain number of phases (such as a six-phase motor) and subsequently eliminating the circulation channel through electric isolation (equivalent to an isolation transformer) between two three-phase windings of the six-phase motor. However, in the above mentioned solutions the isolation transformer has a large volume, which not only increases the cost of the system, but also greatly reduces the power density of the system. A further solution involves special requirements for the motor during elimination of the circulation channel. However, such solution can not be used in different situations.
In view of this, skilled in the art are endeavoring to find ways in which to design a novel converter system, so as to effectively solve the problem of circular current when multiple converter systems are connected in parallel to thereby increase the reliability of the system and reduce the cost of the system.